This invention relates to the use of X-ray or Gamma-ray radiation in the measurement of sheet thickness in ferrous and nonferrous metals. The invention is particularly suitable for use in rolling mills where relatively thin sheet material moves at relatively high speeds and it is desired to provide a continuous measurement of the thickness without contacting the sheet.
Radiation transmission systems have been in use for many years for measuring sheet thickness. A typical system includes a radiation source and a radiation detector positioned on opposite sides of the sheet, with the thickness being a function of the material absorption coefficient of the material at the energy band of the source and the intensity of radiation at the detector. The prior art systems have been reasonably accurate when the composition of the material is known. However if the material composition differs substantially from the nominal or standard composition, measurement errors in the range of one percent to ten percent occur. This is a particular problem when the sheet is rolled from a continuously cast ingot, since, during change from one nominal alloy to another, the composition of the ingot will vary considerably from either of the nominals.
Beta gauges have been used in some sheet and foil mills. Unfortunately, however, sensitivity to oil film, oil drops, air density and gauge misalignment have made the Beta gauges unsatisfactory in rolling operations for thickness measurement.
U.S. Pat. No. 3,121,166 discloses the alternate use of two X-ray sources in measuriing paper web density and can filling. In one embodiment, two different radiations are alternately directed through the paper web, with one radiation transmission providing a measure of the quantity of water and the other providing a measure of the quantity of pulp in the web. Another embodiment uses the alternating source directed through a can with alternating detector outputs coupled into a gain control or nonlinear networks which compensate for the thickness of the can lids, with the output indicating the quantity of material in the can.
These various prior art devices have not been satisfactory for the measurement of thickness of a sheet in a rolling mill with the desired accuracy of .+-.0.25% where the composition of the sheet changes. By way of example, in an aluminum alloy containing copper or zinc a change in the amount of copper or zinc in the order of .+-.0.5% often occurs. Using the conventional equation for calculating thickness as a function of radiation transmission and material absorption coefficient, errors in measured thickness in the order of .+-.5% will occur.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for continuous measurement of sheet thickness without requiring contact with the sheet and with the measurement substantially independent of changes in material composition.